I had forgotten about this Heinlein juvenile completely thinking I was reading it for the first time, but part way in I realized that I had read it before. The human race has finally created a source of propulsion for ships that can offer constant boost and carry its fuel. That means it's "time for the stars." Population pressure and the sort of intrepid adventurousness Heinlein always so brilliantly portrays drives our heroes out in ships pushing the speed of light knowing that relativistic effects will have them return eventually to earth after many decades have passed leaving their world immeasurably changed.
As light years stack up, both transmission time and the energy required for transmitting become intractable. This provides a way to have our heroes cut off and return to strangeness. Instead, in Time For The Stars, Heinlein allows communication by positing telepathic abilities that are instantaneous between some identical twins. The Long Range Foundation pays high fees to incent twin pairs to split up, one going on ship and one staying home. This sets the scene for taking two teenage boys who are as close as any are likely to be and first putting a spike in the relationship by offering just one the dream of a lifetime, being among the first to get to explore the stars, provided the other stays home, and then have them linked and maintain their relationship over a few years for one and a long lifetime for the other. You get to see the relationship evolve as one member of the relationship lives out his teen years and the other has great-grandchildren.
It's a gripping story, full of action like all the Heinlein juveniles, but also has touching relationship components, sometimes missing from those stories. I enjoyed it thoroughly.